i965: Select DF type for 64-bit integers on Gen < 8.

i965: Select DF type for 64-bit integers on Gen < 8.

Gen8 adds Q/UQ types. We attempted to change the types back to DF in the
generator (commit c95380c40), but an assertion added in the FP64 series
(commit e481dcc3) triggers before that code has a chance to execute.

In fact, using Q/UQ in the IR and then changing to DF in the generator
would not work in the presence of source modifiers, etc.

Fixes: d6fcede6 ("i965: Return Q and UQ types for int64 and uint64")
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>